Described herein is apparatus for preventing the jamming of radar by an electronic countermeasure (ECM) jamming or interference signal; and more particularly, apparatus for receiving two or more different polarizations of the radar return signal and the interference signal, and using that different polarization received signal to cancel the interference signal without also cancelling the desired radar return signal.
It is common for radar systems to be jammed by hostile electronic countermeasure interference signal sources. These electronic countermeasure systems typically monitor the radar band, accurately determine the frequency, pulse repetition rate another radar system characteristics, and transmit interfering signals of sufficient power and of appropriate timing to render the radar inoperative. These countermeasure systems operate in real time so that a change of radar frequency will instantaneously be followed by a change of interference frequency. Therefore, some method of interference avoidance is required what will act to cancel interference, both pulsed and continuous, even at the exact frequency of the radar system.
The prior art includes U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERROR CORRECTION SYSTEM, commonly assigned, which cancels an interference signal by producing a correction signal of equal frequency and amplitude but of appropriate polarity. When the interference, correction and desired signals are received and summed, the interference signal is cancelled and the desired signal remains. This system is useful where the desired and interference signals are of the same frequency and are coherent but differ in phase. An example is an aircraft receiving an ILS signal directly from a transmitter and simultaneously receiving a reflected ILS signal from a nearby structure.
A system employing this principle may employ a circuit for varying the basic signal phase and amplitude to produce a correction signal. Such a circuit is described in U.S. Pat. No. 4,016,516, REFLECTIVE SIGNAL CONTROLLER, commonly assigned. This signal controller is designed to be inserted into the path between a source and the utilization device to allow the control of signal amplitude ratio and polarity.
The prior art thus recognizes the problem of interfering signals of the same frequency, and describes the generation of a correction signal of appropriate phase to cancel said interference. This is possible since the correction signal generator is coupled to, and therefore is coherent with the interference signal source.
In the case where the interference signal is produced by an electronic counter measure source (ECM), however, there can be no coupling to the source to generate a coherent correction signal, and cancellation of a jamming signal is not feasible by this method.
An alternative is to cancel such interference by taking advantage of the difference in signal polarization between desired and interference signals. This technique uses circuits equivalent to those required by prior cancellation systems, but requires a different antenna installation. In the case where the interference and desired signals do not have polarization differences, provision is made to change the polarization of the desired signal.